This invention relates to ignition cable assemblies and more specifically to ignition cable assemblies that have a terminal attached to the end of the ignition cable and encased within a flexible elastomeric nipple or boot that provides the primary environmental seal and dielectric insulation for the terminal when it is plugged onto a spark plug.
The primary function of such an ignition cable assembly is to maintain an adequate mechanical, electrical and dielectric connection of the ignition cable to the spark plug while allowing repetitive removals and reconnections for spark plug changes.
Traditional ignition cable assemblies have a number of shortcomings. The terminals are prone to deformation and damage by repeated engagement and disengagement with the terminal of the spark plug. This results in reduced engagement capacity and possible microarcing due to insufficient terminal contact. The microarcing in turn leads to wear and premature failure of the cable conductor.
Another problem is that the force required to disconnect the terminal can increase in use at engine operating temperatures so that the disconnect force exceeds the terminal to cable retention force. This results in terminal pull off when the ignition cable assembly is disconnected for spark plug replacement.
Another problem is that the elastomeric boot can bond to the ceramic insulator of the spark plug after some period of use in an engine operating temperature environment. This results in the elastomeric boot being susceptible to damage when the ignition cable assembly is disconnected. Possible known solutions for this problem include ribbed insulator geometries and interface lubricants and coatings, such as powders, greases, glazes, etc. However, these solutions are not entirely satisfactory, particularly in high operating temperature environments and extended service interval situations.
Another problem associated with the general type of ignition cable assembly discussed above is degradation of the electrical seal between the boot and the insulator of the spark plug. This results from the loss of seal pressure due to compression set of the elastomeric boot and/or tearing of the elastomeric boot upon removal due to adhesion to the insulator of the spark plug. A proposed solution to this problem is the use of ribs on the insulator of the spark plug. This increases the distance from the spark plug terminal to the spark plug shell thereby increasing the voltage required for arcing from terminal to shell. However, this solution produces air gaps or passages between the boot and the insulator that connect interior spaces of the boot with the environment. At system operating voltages, air in these spaces and passages is ionized resulting in corona discharge along the interface between the boot and the insulator which can lead to eventual dielectric breakdown or punctures of the boot. This in turn limits the amount of voltage that can be applied to the ignition cable assembly and spark plug.
Another problem is that the cable terminations and the terminals of the prior art ignition cable assemblies have irregular geometries which create high potential gradients that lead to high E-field intensities.